Breakover angle

Also called ramp breakover angle, ramp angle, rampover angle

Breakover angle is the sharpest ridge a vehicle can drive over without grounding out in the middle, measured between the two tire contact patches and the lowest point of the chassis between them.

What is breakover angle?

Draw a line from the front tire's contact patch up to the lowest point under the middle of the vehicle, and another from there down to the rear tire's contact patch. The angle those two lines make is the breakover angle. It is what decides whether you crest a ridge or sit on it with all four wheels spinning in the air, which is what people mean by high-centred.

It is the least talked about of the three geometry numbers and often the one that stops you first, because unlike approach and departure it is dominated by something you cannot change.

The formal definition is awkwardly worded because it is defined backwards. US federal law calls it the supplement of the largest angle that can be formed by two lines tangent to the front and rear static loaded radii arcs and intersecting at a point on the underside of the automobile. Supplement, meaning 180 degrees minus that angle, which is why a flat-bellied long truck has a breakover angle close to zero and a short stubby one has a large number.

The name changes depending on who is writing. Breakover angle is the American term and the one in the federal rules; SAE J1100 calls it ramp breakover angle, dimension A147; ISO 612:1978 and European law just call it ramp angle, at paragraph 6.9. The EU wants at least 20 degrees before it will classify a light vehicle as off-road. The US fuel economy rules ask for only 14, which tells you something about how hard this one is to achieve on a vehicle anyone actually sells.

Wheelbase is the variable

The longer the wheelbase, the shallower the angle, all else equal. This is why a short-wheelbase Wrangler walks over terrain that beaches a crew-cab long-bed truck with identical ground clearance and identical tires. It is also why a full-size four-door truck with a camper is a different vehicle off-road from the same truck as a two-door short bed, and no amount of lift closes that gap.

The things you can change are the lowest point in the middle, usually the transfer case, the transmission crossmember or a fuel tank, and the tire diameter. Bigger tires lift the whole belly relative to the contact patches, which is why tire size does more for breakover than a body lift does.